Heat exchangers are an important part of many plants and systems, especially refrigeration or heat pump systems. These heat exchangers and their efficiency are of crucial importance in such systems, and it is therefore important to monitor functioning of the heat exchangers to be able to detect abnormality of the heat exchanger, so measures can be taken to remedy any defects.
By abnormality of the heat exchanger is meant that the heat exchanger does not exchange as much energy as expected, i.e. the fluids do not experience the cooling or heating they should. This may be due to fouling of heat exchanger, in that a layer of scale, dirt or grease is deposited on the heat exchanging surface or surfaces leading to reduced heat exchange, as this layer will usually act as an insulating layer. Another possibility is that there is insufficient fluid flow because of dirt or the like blocking or restricting flow through the heat exchanger. Both situations lead to higher power consumption, because the system must work at a higher load than a system working with heat exchangers within the normal range. Further in the event of adverse working conditions with high heat exchange demand and a relatively small temperature difference between the fluids, it may be impossible to meet the demand, which in some systems may have devastating effect.
Often the abnormality will not be detected before an adverse working condition is experienced, in that the demand cannot be met, e.g. leading to an increase of the temperature of a system, which should be kept at a specific temperature. An example of such a system is a refrigerated display cabinet in a shop, where strict legislation in most countries prescribes that when food is not kept below a maximum temperature, it must be discarded, which of course is expensive and devastating for the business. Likewise large computer systems are often kept in air-conditioned rooms, as an excessive temperature may increase the risk of a computer crash, which may entail a high risk of data loss and lost man-hours.
Common provisions for detecting abnormality of a heat exchanger include basic visual inspection at regular intervals to check for dirt at the inlet of the heat exchanger. Often the heat exchangers are placed so inspection is difficult, and hence such inspection is labour consuming. Further an abnormality may arise at different intervals and quite quickly, e.g. in the event of material blocking the inlet to the heat exchanger. This means that to provide a reasonable degree of security against heat exchanger abnormality, it is necessary to inspect the heat exchangers often. Further a visual inspection of the outside of the heat exchanger may not be effective in assessing whether the internal heat exchanging surfaces are subject to fouling etc. causing a reduced heat exchange.
Another known way of detecting abnormality of a heat exchanger is by direct flow measurement. A direct flow measurement requires delicate and expensive equipment, such as hot wire anemometers or the like, and a plurality of flow measurement devices should be used to gain useful information on the overall flow field. It has also been proposed to assess the flow based on pressure sensors, but such pressure sensors are also expensive, and to gain useful information on the overall flow field a plurality of pressure sensors should be used. A further disadvantage of these methods are that they can only be used to establish if there is restricted flow in the heat exchanger, not the situation where the flow is normal, but heat exchange is reduced, e.g. because of fouling of the heat exchanger surface.